1. Field of the Invention
The present invention relates to monitoring the condition of a motor driven system and, more particularly, to method for monitoring the operating condition of different components of a system through use of signature analysis of the motor current.
2. Description of the Prior Art
Numerous motor driven systems involve periodic motion. For example, motor driven gear systems involve periodic motion and may include various periodicities or frequencies characteristic of the different rotating components or gears forming the system. Various known techniques may be used to measure the characteristics of periodic motion, including in particular detection of variations in repetitive patterns. Small, low-cost accelerometers allow efficient measurement of mechanical vibrations that occur in known directions. In addition, numerous voltage dividers, current transducers, and high gain antennas are available for measuring electrical voltages and currents as well as electric and magnetic fields, including electromagnetic waves.
The signals obtained from system measurements are generally analog signals which are digitized for analysis. Fast digitization of electrical signals can be accomplished with digitizing oscilloscopes and with low-cost analog-to-digital (A/D) converters installed in inexpensive, portable computers. In addition, it is now possible to perform extensive digital signal processing (DSP) with available hardware and software, such as by means of a laptop computer using signal-analysis software.
Motor current signal analysis is a cost effective and non-intrusive method for monitoring the condition of rotating motor driven systems. The operational conditions of rotating equipment, such as a motor driven gear system can be analyzed and related to the maintenance needs of the equipment. In analyses of this type, mechanical load variations in the operating components of the system translate into a variation of the current required to drive the motor. This effect is particularly strong in systems driven by small DC motors and becomes weaker with increasing rotor mass. The variations in current are evident as amplitude modulators or oscillations of the current waveform wherein a substantial and relatively predictable oscillation is associated with each segment switching occurrence of a DC motor, and additional, typically smaller, oscillations in the current waveform are produced as a result of characteristics of the driven system components.
In systems utilizing DC motors, more so than AC driven systems, the speed of the motor will vary somewhat in response to variations in torque load on the motor. As a result of such speed variations, the current waveform produced by the motor is not time stable. Consequently, the spectral content of signals produced by the motor is expansive, resulting in a distribution of energy about an average peak energy. Such a distribution of energy makes it difficult to identify specific frequencies of system components having characteristic frequencies which are close to each other.
U.S. Pat. No. 4,965,513 to Haynes et al. discloses a method for motor signature analysis. The method includes separating out frequency and amplitude components of sensed motor current noise and the source of various changes in load are identified, such as periodic gear mesh loading, friction events at frequencies corresponding to the origin of the events, and other motor load varying characteristics. Motor current noise signatures taken at different times over the operating life of the system are compared to detect aging, wear or abnormal operating characteristics.
U.S. Pat. No. 5,461,329 to Linehan et al. discloses a motor current spectrum analysis method for detecting potential failures of rotating equipment in which spectral frequency resolution is enhanced by collecting a sampled data set completely filled with an exact whole number of stationary carrier waves. The method minimizes the frequency distribution of discrete spectral components in a carrier wave of a known average frequency due to the carrier wave and its components. The method is accomplished by adjusting the sampling rate in such a way that an exact whole number of carrier wave cycles is provided, and including providing an adjustable frequency clock which adjusts its output frequency with the frequency variations of a non-stationary analog carrier wave being analyzed.
While the prior art includes methods for analyzing motor driven systems on the basis of information obtained in the time domain and converted to the frequency domain, such methods are generally applicable to monitoring systems operating at substantially constant speed, such as AC motor driven systems, or by implementing an adjustment in the time domain, i.e., by adjusting a sampling rate, to compensate for speed variations.